(1) Field of the Invention
The present invention relates to a polyester vessel having an improved dimension stability. More particularly, the present invention relates to a polyester vessel having an excellent dimension stability and a good appearance characteristic, in which deformation at a high temperature or under a high pressure or vacuum is prevented.
(2) Description of the Prior Art
A vessel obtained by forming a bottomed preform of the amorphous state from a thermoplastic polyester such as polyethylene terephthalate and stretch-drawing the bottomed preform in the axial direction at a drawing temperature of the polyester while blow-drawing the preform in the circumferential direction is excellent in such properties as transparency, impact resistance, gas barrier property, rigidity and pressure resistance, as disclosed in the specification of U.S. Pat. No. 3,733,309, and this vessel is widely used as packaging vessels for various liquids.
However, a polyester vessel for this type is still insufficient in the heat resistance. For example, when the vessel is allowed to stand still in an atmosphere maintained at 60.degree. to 70.degree. C. for 3 to 5 minutes, a volume shrinkage of 1 to 3% is observed, and at a higher temperature, the vessel is thermally deformed to such an extent that the vessel cannot be practically used. This tendency is prominent in the neck portion where the polyester is substantially unoriented and in the amorphous state. More specifically, in the neck portion of the vessel, a screw portion or step portion is formed so as to engage a lid member with the vessel tightly and produce assured sealing, and if this screw portion or step portion has no sufficient rigidity or is poor in the dimension stability, a desirable sealing pressure cannot be maintained.
Various proposals have heretofore been made to heat-set a draw-blown vessel for eliminating the above defects, as shown, for example, in Japanese Patent Application Laid-Open Specifications Nos. 82366/76, 107357/76, 78268/78, 78267/78, Nos. 71/79 and 41973/79. However, these proposals are still insufficient because the obtained heat stabilizing effect is relatively low and troublesome treatments are necessary.
As means for improving the rigidity and dimension stability of the neck portion of the vessel, there is known a method in which the amorphous neck portion is whitened (crystallized) by a heat treatment, as disclosed in Japanese Patent Application Laid-Open Specification No. 53566/76. According to this method, however, only the neck portion is whitened and the transparency is lost in the neck portion, resulting in reduction of the commercial value.
Furthermore, there is known a method in which a polyester vessel is irradiated with radiant rays, as disclosed in Japanese Patent Application Laid-Open Specification No. 53-125184. However, no satisfactory improvement of the dimension stability is attained only by irradiation with radiant rays.